If you reach content on SRP that you do not have a permission to access, this is just that.

It could be for dozens of reasons from limitations to who is allowed to view it (e.g. in case of policy violations only the moderators can see the problematic post), to broken/obsoleted software (in the case of the link in question).

Sometimes this is fixable relatively easy, sometimes (in this case) it is not (it's been like this for 2.5 years). Sorry.